Repeaters, distributed antenna systems, and similar signal repeating systems are wireless communication systems that are used to extend coverage into areas where the radio frequency (RF) signal penetration from traditional base transceiver stations (BTS's) is limited or not present. Those low signal or no signal areas might be inside buildings, in tunnels, shadowed areas that are behind mountains, underground train systems, and various other isolated areas. Generally, applications for such repeater communication systems are for those situations where the repeater or distributed antenna system (DAS) is immobile and is mounted in a fixed location with respect to one or more base transceiver stations. In other applications, the area that has limited penetration of the RF signals is mobile. That is, the repeater or distributed antenna system is installed in a moving or mobile environment or conveyance such as a train, ship, car, bus, or airplane, for example.
One common mobile application for repeater systems is in a train car where the repeater system is used to compensate for the train's signal penetration loss. A typical train car repeater system includes a donor antenna positioned or mounted on the outside of the train car to provide a radio link with a donor site, such as a nearby base transceiver station, and a coverage antenna positioned in the interior of the train car to provide a radio link with mobile devices located in the extended coverage area inside the train car. The donor and coverage antennas are connected by a bi-directional amplifier that boosts the levels of the uplink and downlink radio signals handled by the repeater system so that the signals have sufficient strength to ensure that train passengers can use smart phones and other mobile devices without dropping calls and with the benefit of higher data rates.
Because repeater systems increase the level of uplink signals (e.g., signals from the mobile devices to the base station) through electronic amplification, the repeater system may also generate and transmit spurious signals due to intermodulation distortion caused by non-linearities in the bi-directional amplifier, for example. These spurious signal emissions, and in particular third order intermodulation product emissions, must be controlled to avoid interfering with other mobile communication systems. Third order intermodulation products will often create co-channel interference in adjacent frequency bands. This co-channel interference will usually result in the affected communication system needing a higher received signal level for the particular desired signal than would normally be necessary in order to function properly. Co-channel interference therefore, effectively desensitizes the affected communication systems. For this reason, the allowable out-of-band spurious emissions of wireless repeater systems are regulated by standards bodies such as the European Telecommunications Standards Institute (ETSI) and the 3rd Generation Partnership Project (3GPP) to ensure that such spurious emissions are kept below a desired level.
In a stationary environment, such a consideration takes advantage of the somewhat static signal conditions. Interference control requirements are typically met through proper selection of system components and configuration settings made at the time the repeater system is commissioned. For example, a stationary repeater system providing extended coverage for a distant BTS may use a directional donor antenna that is oriented toward a more distant donor BTS to reduce emissions in the direction of the nearby non-donor BTS.
However, in a mobile environment, the conditions are more dynamic. For example, the positions of donor and non-donor BTS's change with respect to the repeater system, as the train or other mobile platform moves. Therefore, a repeater system configuration that provides acceptable performance at one location of a moving mobile platform may cause unacceptable interference at another location. The mobile environment also places additional constraints on the repeater system design, since the repeater system must operate under changing and somewhat unpredictable environmental conditions.
One specific issue faced by in-train repeater systems involves desensitization of Global System for Mobile communication—Railway (GSM-R) base stations by a mobile repeater system. GSM-R is a secure signal platform that provides voice and data communication between train and railway communication centers. GSM-R is used by railway operating staff such as train drivers, engineers, dispatchers, shunting team members, and station controllers to provide a reliable method of communication. GSM-R base stations are typically deployed along the rail track right-of-way, so the train cars will occasionally pass within a few meters of various GSM-R base stations, as they travel on the track. In configuring such systems, GSM-R operators generally do not allow other GSM-900 service providers to co-locate base transceiver stations on the same towers or structures supporting GSM-R equipment. Therefore, when a train having an in-train repeater system passes a GSM-R base station along a track, the donor base station that is providing GSM-900 service to the riders in the train car is typically much further away from the donor antenna of the repeater system than the GSM-R base station. In a worst-case (but not uncommon) scenario, the in-train repeater system will be near the signal coverage edge or limit of the donor GSM-900 base station as the train passes the GSM-R base station. Therefore, the mobile repeater system will be operating at maximum signal gain in order to maintain signal contact with the donor base station. In such a scenario, the uplink signals transmitted by the in-train repeater system will be much stronger at the closer GSM-R base station than at the more distant donor base station.
This “near-far” condition for the mobile repeater system places greater than normal suppression requirements on the spurious emissions of an in-train repeater system. For this reason, an in-train repeater system that meets the suitable ETSI and 3GPP emissions standards may still cause significant interference with or desensitization of the near GSM-R base station. The spurious emissions of the in-train repeater system may therefore have to be suppressed even more than would normally be required in other types of repeater systems to avoid interfering with the much closer GSM-R base station. This problem is further aggravated due to the mission-critical nature of GSM-R communications, which increases the importance of reducing interference, or the possibility of interference, with GSM-R systems caused by the mobile repeater system.
Therefore, there is a need for improved systems and methods for reducing desensitization of certain base stations, such as GSM-R base stations, by in-train or other mobile repeater systems.